Rate-adaptive therapy with sensor cross-checking

ABSTRACT

A method and system for automatically adjusting the operating parameters of a rate-adaptive cardiac pacemaker in which maximum exertion levels attained by the patient are measured at periodic intervals and stored in order to compute or update a maximum exercise capacity. The slope of the rate-response curve is then adjusted to map an exertion level corresponding to the updated maximum exercise capacity to a maximum allowable pacing rate. In accordance with the invention, a maximum exercise capacity is determined by cross-checking periodic maximum exertion level sensor values with a motion-level sensor value.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This patent application is a continuation of U.S. patent applicationSer. No. 09/638,975, filed on Aug. 15, 2000, now U.S. Pat. No.5,519,495, the specification of which is incorporated herein byreference.

FIELD OF THE INVENTION

This invention pertains to systems and methods for cardiac rhythmmanagement. In particular, the invention relates to a system and methodfor automatically adjusting the operating parameters of a rate-adaptivecardiac pacemaker.

BACKGROUND

A conventional cardiac pacemaker is an implantable battery-poweredelectronic device that responds to sensed cardiac events and elapsedtime intervals by changing its functional states so as to properlyinterpret sensed data and deliver pacing pulses to the heart atappropriate times. The pacing pulses are delivered through a lead madeup of electrodes on a catheter or wire that connects the pacemaker tothe heart. Modern pacemakers are typically programmable so that they canoperate in any mode which the physical configuration of the device willallow. Such modes define which heart chambers are paced, which chambersare sensed, and the response of the pacemaker to a sensed P wave or Rwave. A three-letter code is used to designate a pacing mode where thefirst letter refers to the paced chamber(s), the second letter refers tothe sensed chamber(s), and the third letter refers to the response.Additional sensing of physiological data allows some pacemakers tochange the rate at which they pace the heart in accordance with someparameter correlated to metabolic demand. Such pacemakers, which are theprimary subject of the present invention, are called rate-adaptivepacemakers.

The most common condition for which pacemakers are used is the treatmentof bradycardia. Permanent pacing for bradycardia is indicated inpatients with symptomatic bradycardia of any type as long as it islikely to be permanent or recurrent and is not associated with atransient condition from which the patient may recover.Atrio-ventricular conduction defects (i.e., AV block) that are fixed orintermittent and sick sinus syndrome represent the most commonindications for permanent pacing. In chronotropically competent patientsin need of ventricular pacing, atrial triggered modes such as DDD or VDDare desirable because they allow the pacing to track the physiologicallynormal atrial rhythm, which causes cardiac output to be responsive tothe metabolic needs of the body. Atrial triggering modes arecontraindicated, however, in patients prone to atrial fibrillation orflutter or in whom a reliable atrial sense cannot be obtained. In theformer case, the ventricles will be paced at too high a rate. Failing tosense an atrial P wave, on the other hand, results in a loss of atrialtracking which can lead to negative hemodynamic effects because thepacemaker then reverts to its minimum ventricular pacing rate. Inpacemaker patients who are chronotropically incompetent (e.g., sinusnode dysfunction) or in whom atrial-triggered modes such as DDD and VDDare contraindicated, the heart rate is determined solely by thepacemaker in the absence of intrinsic cardiac activity. That heart rateis determined by the programmed escape intervals of the pacemaker and isreferred to as the lower rate limit or LRL.

Pacing the heart at a fixed rate as determined by the LRL setting of thepacemaker, however, does not allow the heart rate to increase withincreased metabolic demand. Cardiac output is determined by two factors,the stroke volume and heart rate, with the latter being the primarydeterminant. Although stroke volume can be increased during exercise,the resulting increase in cardiac output is usually not sufficient tomeet the body's metabolic needs unless the heart rate is also increased.If the heart is paced at a constant rate, as for example by a VVIpacemaker, severe limitations are imposed upon the patient with respectto lifestyle and activities. It is to overcome these limitations andimprove the quality of life of such patients that rate-adaptivepacemakers have been developed. Rate-adaptive pacemakers operate so asto vary the lowest rate at which the heart is allowed to beat inaccordance with one or more physiological parameters related tometabolic demand.

The body's normal regulatory mechanisms act so as to increase cardiacoutput when the metabolic rate is increased due to an increased exertionlevel in order to transport more oxygen and remove more waste products.One way to control the rate of a pacemaker, therefore, is to measure themetabolic rate of the body and vary the pacing rate in accordance withthe measurement. Metabolic rate can effectively be directly measured by,for example, sensing blood pH or blood oxygen saturation. Practicalproblems with implementing pacemakers controlled by such directmeasurements, however, have led to the development of pacemakers thatare rate-controlled in accordance with physiological variables that areindirectly reflective of the body's metabolic rate such as bodytemperature, ventilation rate, or minute ventilation. Minute ventilationvaries almost linearly with aerobic oxygen consumption during exerciseup to the anaerobic threshold and is the physiological variable that ismost commonly used in rate-adaptive pacemakers to reflect the exertionlevel of the patient.

An even more indirect indication of metabolic rate is provided by themeasurement of body activity or motion. Body activity is correlated withmetabolic demand because such activity requires energy expenditure andhence oxygen consumption. An activity-sensing pacemaker uses apiezoelectric sensor or accelerometer inside the pacemaker case thatresponds to vibrations or accelerations by producing electrical signalsproportional to the patient's level of physical activity.

In such rate-adaptive pacemakers that vary the pacing rate in accordancewith a measured exertion level, the control system is generallyimplemented as an open-loop controller that maps a particular exertionlevel to one particular target heart rate. The mapping is accomplishedby a rate-response curve which is typically a linear function (i.e., astraight line), but could also be some non-linear function as well suchas a dual-slope curve or exponential curve. The rate-response curve isthen defined with minimum and maximum target heart rates. A minimumtarget heart rate for a patient can be ascertained clinically as a heartrate adequate to sustain the patient at rest, while a maximum allowabletarget heart rate is defined with a formula that depends on thepatient's age. The rate-response curve then maps a resting exertionlevel to the minimum heart rate and maps the maximum exertion levelattainable by the patient, termed the maximum exercise capacity, to themaximum allowable heart rate. The responsiveness of the control system,defined as how the target heart rate changes with a given change inexertion level, depends upon the slope of the rate-response curve (orslopes in the case of a dual-slope curve) which is dictated by thedefined maximum exercise capacity. If the maximum exercise capacity isincorrectly defined, the pacemaker's responsiveness will not be set toan appropriate level. An under-responsive pacemaker will unnecessarilylimit exercise duration and intensity in the patient because the heartrate will not increase enough to match metabolic demand, while anover-responsive pacemaker can lead to palpitations and patientdiscomfort.

In order to define a patient's maximum exercise capacity, exercisetesting can be performed to determine the maximum exertion level thatthe patient is capable of attaining. The pacemaker can then beprogrammed with that value with adjustments made during follow-upclinical visits. Exercise testing may not always be practical, however,and a patient's maximum exercise capacity can change over time due to,e.g., physical conditioning, illness, or recovery from illness, whichincreases the need for follow-up visits. Algorithms have therefore beendeveloped that attempt to adjust the responsiveness of rate-adaptivepacemakers automatically in accordance with exertion level measurementsmade as the patient goes about ordinary activity. Determining apatient's maximum exercise capacity from periodic exertion levelmeasurements, however, is problematical since it is not known how closeto the true maximum a periodic maximum exertion level is.

SUMMARY OF THE INVENTION

The present invention relates to a method for automatically adjustingthe responsiveness of a rate-adaptive pacemaker by estimating a maximumexercise capacity from periodic exertion level measurements. Inaccordance with the invention, exertion levels in the patient aremeasured by an exertion level sensor to determine a periodic maximumlevel. The periodic maximum exertion level is then cross-checked with asimultaneously taken activity level measurement to form an estimate ofthe patient's maximum exercise capacity.

In one embodiment, exertion levels are measured with a minuteventilation sensor, and activity levels are measured with anaccelerometer, so that each measured minute ventilation may beassociated with a simultaneously taken activity level measurement. Inorder to estimate the maximum minute ventilation attainable, whichcorresponds to the patient's maximum exercise capacity, a maximum minuteventilation together with its paired activity level is determined foreach day (or other time period). The activity level measurementassociated with the periodic maximum ventilation measurement is thencross-checked with the activity level measurement in order to estimatethe maximum exercise capacity. The activity level measurement is mappedto a percentage of minute ventilation reserve, the minute ventilationreserve being the difference between a patient's resting and maximumattainable minute ventilation. The cross-checking may be performed bydividing the periodic maximum exertion level by the percentage of minuteventilation reserve represented by the activity level measurement. Oneor more such estimates are used to estimate the patient's maximumexercise capacity. In another embodiment, pairs of minute ventilationand activity level measurements are validated with previously storedpairs before being used to estimate maximum exercise capacity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a rate-adaptive pacemaker.

FIG. 2 depicts a rate-response curve.

FIG. 3 is a flow chart illustrating a particular implementation of thepresent method.

DESCRIPTION OF SPECIFIC EMBODIMENTS

A particular implementation of a rate-adaptive pacemaker as shown inFIG. 1. As used herein, the term pacemaker should be taken to mean anycardiac rhythm management device with a pacing functionality includingan implantable cardioverter/defibrillator that includes a pacemaker. Apacemaker controller senses cardiac events through a sensing channel andoutputs pacing pulses to the heart via a pacing channel in accordancewith a programmed pacing mode. A microprocessor serves as the controllerin this embodiment and communicates with a memory 12 via a bidirectionaldata bus 13. The memory 12 typically comprises a ROM or RAM for programstorage and a RAM for data storage. The pacemaker has atrial sensing andpacing channels comprising electrode 34, lead 33, sensing amplifier 31,pulse generator 32, and an atrial channel interface 30 whichcommunicates bidirectionally with a port of microprocessor 10. Thedevice also has ventricular sensing and pacing channels comprisingelectrodes 24, leads, sensing amplifier 21, pulse generator 22, andventricular channel interface 20. For each channel, the same lead andelectrode are used for both sensing and pacing. The channel interfaces20 and 30 include analog-to-digital converters for digitizing sensingsignal inputs from the sensing amplifiers and registers which can bewritten to by the microprocessor in order to output pacing pulses,change the pacing pulse amplitude, and adjust the gain and thresholdvalues for the sensing amplifiers. A telemetry interface 40 is alsoprovided for communicating with an external programmer. A minuteventilation sensor MVS and an accelerometer AC are employed to sense theminute ventilation and body activity, respectively. The pacemaker usesthe sensed minute ventilation and/or the accelerometer signal to adjustthe rate at which the pacemaker paces the heart in the absence of afaster intrinsic rhythm. The microprocessor 10 executes programmedinstructions that implement various pacing and rate-adaptive algorithms,including the method for determining maximum exercise capacity asdescribed below.

The responsiveness of the pacemaker is controlled in accordance with arate-response curve RRC as shown in FIG. 2. Other embodiments may use adual-slope curve or a non-linear curve. A change in exertion level asdetermined from a minute ventilation measurement causes a proportionalchange in the target heart rate in accordance with the slope of thecurve, termed the response factor RF. The target heart rate is then usedas a lower rate limit (LRL) by the pacemaker to pace the heart inaccordance with a programmed pacing mode. The LRL is the rate at whichthe heart is paced in the absence of faster intrinsic activity. As shownin the figure, the rate response curve maps a resting exertion level RELto a minimum target rate MinHR which corresponds to the minimum LRL thatis to be used by the pacemaker. The maximum target rate MaxHR is themaximum rate at which the pacemaker is allowed to pace the heart and ismapped to by the rate response curve from the maximum exertion level thepatient is expected to be able to reach, referred to as the maximumexercise capacity MEC. In the single-slope rate response curve shown inFIG. 2, the response factor RF may then be defined as:RF=(MaxHR−MinHR)/(MEC−REL)

The minimum target rate and resting exertion level for a particularpatient can be determined clinically after implantation. The maximumtarget rate can initially be determined from formulas derived frompopulation data and dependent upon the patient's age or chosen by thephysician. The responsiveness of the pacemaker is initially determinedby setting the response factor RF to an initial value (e.g., 5) or bydefining an initial maximum exercise capacity, which determines theendpoints and hence the slope of the rate response curve. An automaticparameter setting mode can then be employed so that the MEC and RF arechanged from the initially set values in accordance with measurements ofmaximum exertion levels taken by the pacemaker. One way to do this is tomonitor the patient's exertion levels, collect daily maximum exertionlevels for a period of time (e.g., a week, a month, or six months), andselect the highest exertion level among the daily maximums as anestimate of the patient's maximum exercise capacity. The rate responsecurve is then changed accordingly. Such a procedure allows therate-adaptive pacemaker to be tuned to the individual patient as he goesabout ordinary activities and also allows the pacemaker to adjust withchanges in the patient's physical conditioning.

The procedure described above for estimating maximum exercise capacityuses a historical record of attained maximum exertion levels in order tolessen the likelihood of underestimation. That is, if the MEC weresimply estimated as the maximum exertion level attained by the patientduring a particular day, and the patient never actually exercisedmaximally during the day, the estimated MEC will be less than its truevalue. Setting the MEC lower than it should be causes the pacemaker tobe over-responsive, the adverse clinical impact of which is potentiallygreater than if the pacemaker is made under-responsive. Estimating theMEC based upon historical measurements, however, necessarily delays thetime before the responsiveness of the pacemaker can be adjusted.

In accordance with the present invention, an activity level measurementtaken with a motion or pressure sensor such as an accelerometer isassociated with a simultaneously taken exertion level measurementdetermined to be a maximum over a specified period. The periodic maximumexertion level measurement is then cross-checked with the associatedactivity level measurement to estimate the patient's maximum exercisecapacity. A mapping based upon either population data or an assessmentof the individual patient relates a particular activity level to aparticular percentage of the patient's reserve exercise capacity. Thereserve exercise capacity is defined as the difference between themaximum exercise capacity and the exertion level corresponding to rest.For example, a linear mapping may be used with a no-activity measurementvalue mapped to 0 percent of the reserve (i.e., the resting exertionlevel), a maximum activity measurement value mapped to 100 percent ofthe reserve (i.e., the maximum exercise capacity), and linearlyinterpolated values therebetween. The maximum exercise capacity is thenestimated from the periodic maximum exertion level measurement basedupon the percentage of the reserve exercise capacity that the associatedactivity level measurement corresponds to.

In one embodiment, for example, a minute ventilation sensor measuresexertion levels of the patient and periodically (e.g., daily) records amaximum exertion level. Each recorded daily maximum exertion level isassociated with an activity level measurement simultaneously taken withan accelerometer. The activity level measurement is mapped to apercentage of the patient's reserve exercise capacity, also referred toin this case as the minute ventilation reserve. The measured dailymaximum minute ventilation is then cross-checked with the activity levelmeasurement to estimate the patient's maximum exercise capacity. Thecross-checking may be performed by dividing the measured maximum minuteventilation by the percentage of minute ventilation reserve representedby the activity level measurement. An updated response factor may thenbe computed from the estimate of maximum exercise capacity or from anaverage such estimates. If a minimum degree of responsiveness isdesired, a minimum value for the response factor can be defined, or aminimum value can be defined for the percentage of reserve exercisecapacity represented by an activity level measurement.

Alternatively, the cross-checking of the periodic maximum exertion levelmay be performed with discrete percentages of reserve exercise capacityaccording to threshold values of measured activity levels. A range ofmeasured activity level values then dictates the percentage that theperiodic maximum exertion level is to be divided by in order to estimatethe maximum exercise capacity. In an exemplary embodiment of the methodillustrated in FIG. 3, two activity level thresholds are set ataccelerometer measurements of 60 mg and 100 mg, representing minuteventilation reserves of 50 percent and 75 percent, respectively. At stepS1, minute ventilation and accelerometer activity level data arecollected during the day. Spurious peaks in the sensor signals can beremoved with a moving average filter (e.g., 1 minute averages). At stepS2, a daily maximum minute ventilation and an associated activity levelare determined. Steps S3 and S5 test the activity level against thethreshold activity level values. If the accelerometer measurement isgreater than 100 mg, the maximum exercise capacity is estimated to bethe same as the daily maximum minute ventilation at step S4. If theaccelerometer measurement is greater than 60 mg but less than 100 mg,the maximum exercise capacity is estimated to be the daily maximumminute ventilation multiplied by 1.33 (i.e., divided by 75 percent) atstep S6. If the accelerometer measurement is not greater than 60 mg, themaximum exercise capacity is estimated to be the daily maximum minuteventilation multiplied by 2.0 (i.e., divided by 50 percent) at step S7.

In another other embodiment, the maximum exercise capacity is estimatedas the maximum among a plurality of historical daily maximums. The dailymaximums are validated before being cross-checked with associatedactivity levels. A daily maximum and its associated activity level canalso be validated by comparing them with past pairs. The daily maximumand associated activity level can then be used to compute the average ordisregarded as spurious data. Still other embodiments may employ theestimated maximum exercise capacity to adjust the responsiveness ofpacemakers with more complex rate-response curves (e.g., dual-slope orexponential).

Although the invention has been described in conjunction with theforegoing specific embodiment, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Such alternatives, variations, and modifications are intended to fallwithin the scope of the following appended claims.

1. A system, comprising: level sensor for measuring exertion levels; anactivity level sensor; a processor coupled to the exertion level sensorand the activity level sensor; a pulse generator coupled to theprocessor for delivering pacing pulses in accordance with arate-adaptive pacing mode based upon a rate-response curve; and, whereinthe processor is programmed to: determine a resting exertion level fromthe measured exertion levels; determine a maximum exertion level fromthe measured exertion levels; associate the maximum exertion level witha corresponding activity level; estimate a maximum exercise capacity bycross-checking the maximum exertion level with the correspondingactivity level; and adjust a response factor based on the estimatedmaximum exercise capacity and the resting exertion level, the responsefactor being a slope of the rate-response curve in a rate-adaptivecardiac pacing mode.
 2. The system of claim 1, further comprising: anatrial sensing and pacing channel communicatively coupled to theprocessor; and a ventricular sensing and pacing channel communicativelycoupled to the processor.
 3. The system of claim 2, further comprisingat least one moving average filter, coupled to the exertion levelsensor, to remove spurious peaks in signals sensed by the activity levelsensor.
 4. The system of claim 2, further comprises at least one movingaverage filter, coupled to the activity level sensor, to remove spuriouspeaks in signals sensed by the activity level sensor.
 5. The system ofclaim 2, wherein the exertion level sensor comprises a minuteventilation sensor.
 6. The system of claim 2, wherein the activity levelsensor comprises an accelerometer.
 7. A method, comprising: measuringexertion levels; measuring activity levels; determining a restingexertion level from the measured exertion levels; determining a maximumexertion level from the measured exertion levels; associating themaximum exertion level with a corresponding activity level; estimating amaximum exercise capacity by cross-checking the maximum exertion levelwith the corresponding activity level; and adjusting a response factorbased on the estimated maximum exercise capacity and the restingexertion level, the response factor being a slope of a rate-responsecurve in a rate-adaptive cardiac pacing mode.
 8. The method of claim 7,further comprising defining a minimum value for the response factor. 9.The method of claim 7, further comprising determining a maximum targetheart rate and a minimum target heart rate, and wherein adjusting theresponse factor comprises adjusting the response factor based on themaximum target heart rate, the minimum target heart rate, the maximumexercise capacity, and the resting exertion level.
 10. The method ofclaim 9, wherein adjusting the response factor comprises determining theresponse factor using an equation:RF=(MaxHR−MinHR) / (MEC−REL), where RF is the response factor, MaxHR isthe maximum target heart rate, MinHR is the minimum target heart rate,MEC is the maximum exercise capacity, and REL is the resting exertionlevel.
 11. The method of claim 10, further comprising determining theminimum target heart rate and the resting exertion level after animplantation of a rate-adaptive pacemaker.
 12. The method of claim 11,further comprising determining an initial value of the maximum targetheart rate based on population data and age.
 13. The method of claim 7,wherein determining the maximum exertion level comprises determining aperiodic maximum exertion level.
 14. The method of claim 13, whereindetermining the periodic maximum exertion level comprises: collecting aplurality of first maximum exertion levels each determined from exertionlevels measured over a predetermined first period; and selecting ahighest exertion level among the plurality of first maximum exertionlevels collected over a predetermined second period.
 15. The method ofclaim 14, wherein determining the periodic maximum exertion levelcomprises: collecting a plurality of daily maximum exertion levels overa predetermined period; and selecting the highest exertion level amongthe plurality of daily exertion levels.
 16. The method of claim 14,wherein estimating the maximum exercise capacity comprises: estimating aplurality of first periodic maximum exercise capacities bycross-checking each of the plurality of first maximum exertion levelswith its corresponding activity level; and selecting the maximumexercise capacity from the plurality of first periodic maximum exercisecapacities.
 17. The method of claim 7, further comprising removingspurious peaks in at least one of the measured exertion levels and themeasured activity levels.
 18. The method of claim 17, wherein removingspurious peaks in at least one of the measured exertion levels and themeasured activity levels comprises using a moving average filter toremove the spurious peaks.
 19. The method of claim 7, further comprisingmapping the corresponding activity level to a percentage of a reserveexercise capacity being the difference between the maximum exercisecapacity and the rest exertion level, and wherein the cross-checkingcomprises dividing the maximum exertion level by the percentage of thereserve exercise capacity represented by the corresponding activitylevel.
 20. The method of claim 19, furthering comprising defining aminimum value for the percentage of the reserve exercise capacityrepresented by the corresponding activity level.
 21. A systemcomprising: means for measuring exertion levels and determining restingand maximum exertion levels; means for measuring activity levels andassociating the maximum exertion level with a corresponding activitylevel measurement; means for estimating a maximum exercise capacity bycross-checking the maximum exertion level with the correspondingactivity level measurement; means for delivering pacing pulses inaccordance with a rate-adaptive pacing mode based upon a rate-responsecurve; and, means for adjusting a response factor based on the estimatedmaximum exercise capacity and the resting exertion level, the responsefactor being a slope of the rate-response curve in the rate-adaptivecardiac pacing mode.
 22. The system of claim 21, further comprisingmeans for mapping activity levels to percentages of reserve exercisecapacity, and wherein the means for estimating the maximum exercisecapacity comprises means for cross-checking the maximum exertion levelwith the corresponding activity level measurement by dividing themaximum exertion level by the percentage of reserve exercise capacitymapped to by the corresponding activity level measurement.
 23. Thesystem of claim 22, wherein the means for mapping activity levels topercentages of reserve exercise capacity comprises means for mapping theactivity levels to the percentages of reserve exercise capacity by alinear relationship.
 24. The system of claim 22, wherein the means formapping activity levels to percentages of a reserve exercise capacitycomprises means for mapping the activity levels to discrete percentagesof reserve exercise capacity according to predetermined activity levelthresholds.
 25. The system of claim 21, further comprising: means fordetermining a plurality of periodic maximum exertion levels; means forcross-checking the plurality of periodic maximum exertion levels; andmeans for estimating the maximum exercise capacity as a maximum amongthe plurality of cross-checked periodic maximum exertion levels.